Extending Mendelian Genetics

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Q Why are there so many variations among people? It will come as no surprise to you, but you are not a pea plant. But Mendel’s principles apply to you just as they apply to other organisms. About 99.9% of everyone’s DNA is identical. So how can a 0.1% difference in DNA lead to the wide range of human traits? In many organisms, genetics is more than dominant and recessive alleles.

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USING LANGUAGE Your Turn Analogies An analogy question asks you to analyze the Use information in the chapter to complete this analogy. relationship between two words in one pair and to allele : gene :: trait: ______identify a second pair of words that have the same (Hint: Finding out how alleles and genes are related will relationship. Colons are used to express the analogy for help you figure out which word to use to fill in the this type of question. For example, the analogy “up is to blank.) down as top is to bottom” is written “up : down :: top : bottom.” In this example, the relationship between the words in each pair is the same. (tl) ©Getty Images; (tc) ©Blend Images/Alamy; (tr) ©Kaz Mori/Getty Images; (cl) ©Getty Images; (cr) ©James Woodson/Getty Images; (bl) ©Nancy Honey/Getty Images; Images;(bc) ©Hans Neleman/Getty (bl) ©Nancy (br) ©Navaswan/GettyWoodson/Getty Images(tl) ©Getty Images; (tc) ©Blend Images/Alamy;©Getty Images; (tr) ©Kaz Mori/Getty Images;(cr) ©James (cl)

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Hair texture isjust Chromosomes andPhenotype af ph T learning aboutthecomplexities ofgenetics. the dominantandrecessive relationship amongalleles isagood place to when start so manysubtle differences inanyofthose traits? Inmost cases, theanswer isno.But all influenced by genetics. Candominantandrecessive alleles ofonegene produce traits around you. Haircolor andtexture, eye color andshape,height,weight are The next timeyou are inacrowd ofpeople, take amomentto look at thevariety of Your to World Connect Ke wo copies ofeach autosomal affect gene

fect the expression of traits. of expression the fect y homologous chromosome—and each copy can influence phenotype. Why? there are Because two copies of each gene on autosomes—one on each Mendel could predict phenotypes the that would appear plants. inhis pea chance of aperson having one of disorders these predicted, can be just as texture—curly hair or straight hair—are autosomal genes. or straight? What about your parents’ hair? The genes that your affect hair humans, are result the of autosomal genes. at Look somes. In most fact, traits reproducing insexually organisms, including flowers. canalleles produce different phenotypes, such as white flowers or purple have different for alleles genes. those And, as Mendel different observed, parents. chromosomes Both have same the genes, but chromosomes the might chromosome. pair consists Each of one chromosome from each of two chromosomes, and do not they play adirect role determination. insex chromosomessex determine an organism’s Autosomes sex. are of all other the agenewhether is located on an autosome or on chromosome. asex that Recall chromosome upon which agene is located. expression Gene is often related to recessive But alleles. many factors phenotype, affect including specific the You have already how learned some genetic traits on depend dominant and MAI MAI C enotype.

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Disorders Caused by Recessive Alleles Figure 1.2 autosome inheritance Some human genetic disorders are caused by recessive alleles Some genetic disorders, such as cystic fibrosis, are on autosomes. Two copies of the recessive allele must be present inherited according to Mendel’s principles. for a person to have the disorder. These disorders often appear heterozygous parent (Cc), carrier in offspring of parents who are both heterozygotes. That is, each parent has one dominant, “normal” allele that masks the C c one disease-causing recessive allele. For example, cystic fibrosis is a severe recessive disorder that CC Cc C homozygous heterozygous, mainly affects the sweat glands and the mucus glands. A person dominant carrier who is homozygous for the recessive allele will have the disease. heterozygous parent (Cc), Someone who is heterozygous for the alleles will not have the carrier disease but is a carrier. A carrier does not show disease symp- Cc cc c heterozygous, homozygous toms but can pass on the disease-causing allele to offspring. In carrier recessive, aected this way, alleles that are lethal, or deadly, in a homozygous recessive individual can remain in a population’s gene pool. C 5 Normal allele (dominant) This inheritance pattern is shown in figure 1.2. c 5 Cystic ﬁbrosis allele (recessive) Disorders Caused by Dominant Alleles Dominant genetic disorders are far less common than recessive disorders. One example is Huntington’s disease. Huntington’s disease damages the nervous system and usually appears during adulthood. Because the disease is caused by a dominant allele, there is a 50% chance that a child will have it even if only one parent has one of the alleles. If both parents are heterozygous for the disease, there is a 75% chance that any of their children will inherit the disease. Because Huntington’s disease strikes later in life, a person with the allele can have children before the disease appears. In that way, the allele is passed on in the population even though the disease is fatal. Connect How are Mendel’s observations related to genes on autosomes?

MAIN IDEA Males and females can differ in sex-linked traits. Mendel figured out much about heredity, but he did not know Figure 1.3 Sex chromosome inheritance about chromosomes. As it turns out, he only studied traits produced by genes on autosomes. Now we know about sex The gametes from an XY male determine the sex chromosomes, and we know that the expression of genes on the of the offspring. sex chromosomes differs from the expression of autosomal genes. female parent (XX) XX Sex-Linked Genes Genes that are located on the sex chromosomes are called sex-linked genes. Recall that many species have specialized sex X XXXX female female chromosomes called the X and Y chromosomes. In mammals and some other animals, individuals with two X male parent (XY) chromosomes—an XX genotype—are female. Individuals with one X and one Y—an XY genotype—are male. As figure 1.3 XY XY Y male male shows, a female can pass on only an X chromosome to offspring, but a male can pass on an X or a Y chromosome.

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R E ADI N G TO OLB ox Genes on the Y chromosome are responsible for the development of male offspring, but the X chromosome actually has much more influence over TAKING NOTES phenotype. The X chromosome has many genes that affect many traits. Use a two-column chart to compare and contrast the Scientists hypothesize that the Y chromosome may have genes for more than expression of autosomal and sex determination, but there is little evidence to support this idea. sex-linked genes. In many organisms, including humans, the Y chromosome is much smaller autosomes sex chromosomes and has many fewer genes than the X chromosome. Evidence suggests that over millions of years of evolution, the joining of the X and Y chromosomes during meiosis has resulted in segments of the Y chromosome being transferred to the X. You will read more about specific sex-linked genes and their locations on the human X and Y chromosomes in Section 4. Expression of Sex-Linked Genes Because the X and Y chromosomes have different genes, sex-linked genes have a pattern of expression that is different from autosomal genes. Remember, two Biology VIDEO C copies of an autosomal gene affect a trait. What happens when there is only LIP one copy of a gene, as is the case in an XY male? Because males have only one HMDScience.com copy of each type of sex chromosome, they express all of the alleles on both GO ONLINE chromosomes. In males, there are no second copies of sex-linked genes to Sex-Linked Inheritance mask the effects of another allele. This means that even if all of the alleles of sex-linked genes in a male are recessive, they will be expressed.

QU i c K L A B predicting Sex-Linked Inheritance The relationship between genotype and phenotype in sex-linked genes differs from that in autosomal genes. A female must have two recessive alleles of a sex-linked gene to express a recessive sex-linked trait. Just one recessive allele is needed for the same trait to be expressed in a male. In this lab, you will model the inheritance pattern of sex-linked genes. Problem How does probability explain sex-linked inheritance? Procedure Materials 1. Use the tape and marker to label two coins with the genetic • 2 coins cross shown on your group’s index card. One coin represents • masking tape the egg cell, and the other coin represents the sperm cell. • marker 2. Flip the two coins and record the genotype of the “offspring.” • index card with genetic cross 3. Repeat Step 2 until you have modeled 50 genetic crosses. Make a data table to record each genetic cross that you model. 4. Calculate the genotype and phenotype probabilities for both males and females. Calculate the frequency of male offspring and female offspring. Analyze and Conclude 1. Analyze Do all of the females from the genetic cross show the recessive trait? Do all of the males show the recessive trait? Why or why not? 2. Apply Make a Punnett square that shows the genetic cross. Do the results from your Punnett square agree with those from your experiment? Why or why not? (br) © HMH

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7.2 Complex Patterns of Inheritance

6F Key Concept Phenotype is affected by many different factors. MAIN IDEAS VOCABULARY Phenotype can depend on interactions of alleles. incomplete dominance Many genes may interact to produce one trait. codominance The environment interacts with genotype. polygenic trait

Connect to Your World 6F predict possible outcomes of various genetic Suppose you have blue and yellow paints to paint a room. You paint the walls yellow, combinations such as monohybrid let them dry, then paint the walls blue. The blue paint masks the yellow paint, so you crosses, dihybrid crosses and non-Mendelian inheritance could say that the blue paint is “dominant.” You could also combine the paints in other ways. You could paint the room in blue and yellow stripes, or you could mix the colors and paint the room green. You can think of different alleles as different paint colors, but in genetics there are many more paint colors—alleles—and many more ways that they are combined.

MAIN IDEA 6F CONNECT TO Phenotype can depend on interactions of alleles. Principles of Genetics Although Mendel’s basic theory of heredity was correct, his research could not Recall from the chapter Meiosis have explained all of the continuous variations for many traits. For example, and Mendel that a homozygote many traits result from alleles with a range of dominance, rather than a strict has two identical alleles of a dominant and recessive relationship. gene, and a heterozygote has two different alleles of a gene. The pea flowers that Mendel observed were either white or purple. One allele was dominant, but dominance does not mean that one allele “defeats” the other. Usually, it means that the dominant allele codes for a certain protein, and the recessive allele codes for a variation of the protein that has little or no effect. In Mendel’s pea flowers, a heterozygous plant makes enough of HMDScience.com the purple color that only one dominant allele is needed to give the flowers a GO ONLINE purple color. But in many cases, a phenotype comes from more than just one Experiments and Models of gene, and many genes in a population have more than just two alleles. Heredity Incomplete Dominance Sometimes, alleles show incomplete dominance, in which a heterozygous phenotype is somewhere between the two homozygous phenotypes. Neither allele is completely dominant nor completely recessive. One example of incomplete dominance is the four-o’clock plant. When plants that are homozygous for red flowers are crossed with plants that are homozygous for white flowers, the offspring have pink flowers. The pink color is a third, distinct phenotype. Neither of the original phenotypes of the plants in the parent’s

generation can be seen separately in the F1 generation offspring.

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alleles, shownalleles, in consideredalso amultiple-allele trait. The multiple different human inthe alleles population, trait this is come types And, from blood the because three an types—is example blood ABO of codominance. havewill some red areas and some white areas. ate phenotype, both traits are expressed. The flowers are different. Instead of what looks like an intermedi- incomplete dominance, the offspring have pink flowers. Codominant alleles for red flowers is crossed with a plant that is homozygous for white flowers. In traits are and fully separately expressed. Suppose a plant that is homozygous with a genotype of I genotype of I and not does result inan antigen. Someone with a on of surface the iis cells. Allele redrecessive blood all ofall offspring the have heterozygous the genotype (B (25%) will be steel be blue(25%) will (B dominant nor recessive. In this case, alleles show Sometimes, both alleles of a gene are expressed completely—neither allele is Codominance in remember that I the Both I Both (25%) will be green (B be (25%) will happens two when royal blue are fish betta crossed? Some offspring The of alleles gene this follow apattern of incomplete dominance. What aroyal be will blue color that comes from phenotypes the from alleles. both Phenoty figure 2.1. figure One t Another example of incomplete dominance is color the of shown fish betta green A 2.1 and I rait that you likely know about—human

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People with both codominant alleles (IAIB) have both antigens, so they have type AB blood. People with an ii genotype have red blood cells without either antigen, and they have type O blood. Two heterozygous people, one with type A blood (IAi) and one with type B blood (IBi), can have offspring with any of the four blood types, depending on the alleles that are passed on. Apply How can two people with type B blood have a child with type O blood?

MAIN IDEA Many genes may interact to produce one trait. As you have seen, some variations in phenotype are related to incomplete dominance, codominance, and multiple alleles. But most traits in plants and animals, including humans, are the result of several genes that interact. Polygenic Traits Traits produced by two or more genes VISUAL VOCAB are called polygenic traits. Human Traits that are produced by two or more skin color, for example, is the result of genes are called polygenic traits. four genes that interact to produce a many genes continuous range of colors. Similarly, human eye color, which is often poly genic thought of as a single gene trait, is polygenic. As figure 2.3 shows, at least three genes with complicated patterns of expression play roles in determining eye color. For example, the green allele is dominant to blue alleles, but it is recessive to all brown alleles. These genes do not account for all eye color variations, such as changes in eye color over time, the continuous range of eye colors, and patterns of colors in eyes. As a result, scientists hypothesize that still undiscovered genes affect eye color. Epistasis Another polygenic trait is fur color in mice and in other mammals. In mice, at least five different genes interact to produce the phenotype. Two genes give the mouse its general color, one gene affects the shading of the color, and one gene determines whether the mouse will have spots. But the fifth gene involved in mouse fur color can overshadow all of the others. In cases such as this, one gene, called an epistatic gene, can interfere with the expression of other genes. FIGURE 2.3 eye Color At least three different genes interact to produce the range of human eye colors, such as in the examples on the right. Gene Name Dominant Allele Recessive Allele BEY1 brown blue

BEY2 brown blue

GEY green blue

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(t), (b) ©Dwight R. Kuhn Figure 3.1 Figure map linkage VOCABULARY and adifferent bodycolor. (bottom) haswhite eyes, nowings, phenotype. Themutant fruitfly (top to studythegenomes oforganisms and chromosomal analysis are used fingerprinting, genetic modifications, techniques suchasDNA of scientists; 6Hdescribe how history andcontributions ofbiology data; 3Fresearch anddescribe the inferences, andpredict trends from ) shows themost common 2Ganalyze, evaluate, make

7.3 The wild typefruitfly 2G, 3F, 6H Wild type Mutant Gene Linkage andMapping ch Gene linkage wasGene explained flies. through fruit specific places onchromosomes. showed not only that genes are onchromosomes, butalso that genes are found at arekitchen, fruitflies pests.Inthelaboratory, early experiments withfruitflies most useful organisms for genetic research—fruit around flies—buzzing Inyour it. If you leave abananaoutontable untilitisvery ripe,you mightsee some ofthe Your to World Connect Ke Synthesize

y chromosomes must exchange homologous genes during meiosis. genes were not time, Morgan inherited together every concluded also that somes, not genes, the assort independently during linked the meiosis. Because concluded that genes linked were on same the chromosome. The chromo- traits identified by Morgan matches one of chromosome the pairs. Morgan have flies fruit four pairs of chromosomes. of Each four the groups of linked traits traits, linked and appeared they into to fall four groups. As it turns out, pattern. Some traits appeared inherited to together. be Morgan these called th common phenotype. You examples can see in of flies fruit with mutant flies type crossedThey wild the with or adifferent, flies flies, less with traitsfly or associated type, with most wild the common phenotype. up experiments similar to Mendel’s dihybrid crosses. chose They one of type color, and wing shape. Knowing variations, these Morgan and his students set among easily identifiable flies observed fruit variations ineyecolor, body hebecause could quickly and cheaply grow generations new He of flies. for an organism to ingenetic use research. He useful found very flies fruit (Drosophila melanogaster ), found answer. the At first,Morgan was just looking andlinked follow still Mendel’s law of independent assortment? suggested that some genes were together. linked But how could genes be differed from 9:3:3:1phenotype the ratios that Mendel The results observed. Bateson, like Mendel, studied dihybrid crosses plants. of pea But results their Bateson C.Punnett, and R. invented who Punnett the square. Punnett and linkage,Gene which you read about previously, by William was first described romosomes. MAI MAI C e 9:3:3:1ratio predicted by Mendel. But results the did differinanoticeable

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DATA ANALYSIS 2g

Constructing bar graphs Table 1. Drosophila responses to light Scientists tested the reaction of fruit flies to stress by exposing them to bright light—a source of stress for Drosophila. The Strain Condition Half-Time (min) scientists recorded the time it took for half of the flies in each Wild type 1 control 4.0 group to reach food, which they called a “half-time.” Three strains of flies were tested—wild type 1, wild type 2, and a Wild type 1 bright light 12.5 mutant eyeless type—under a controlled condition and with Wild type 2 control 4.5 bright light. The data are shown in Table 1. Wild type 2 bright light 12.0 1. Graph Data Construct a bar graph that shows the data Eyeless control 4.5 in the table. Recall that the independent variable is on the x-axis and the dependent variable is on the y-axis. Eyeless bright light 5.0 2. Analyze How did the condition of bright light affect Source: V. Min, B. Condron, Journal of Neuroscience Methods, 145. the flies? Were all strains affected to the same degree? Why or why not?

MAIN IDEA 3F, 6h Linkage maps estimate distances between genes. The probability that two genes on a chromosome will be inherited together is related to the distance between them. The closer together two genes are, the CONNECT TO more likely it is that they will be inherited together. The farther apart two Crossing Over genes are, the more likely it is that they will be separated during meiosis. Recall from the chapter Meiosis One of Morgan’s students, Alfred Sturtevant, hypothesized that the fre- and Mendel that segments of non-sister chromatids can be quency of cross-overs during meiosis was related to the distance between exchanged during meiosis. genes. This meant that the closer together two genes were, the more likely they were to stay together when cross-overs took place. Sturtevant identified three linked traits in fruit flies—body color, eye color, and wing size—and then crossed the fruit flies. He recorded the percentage of times that the phenotypes did not appear together in the offspring. This percentage represented the frequency of cross-overs between chromosomes. From the cross-over frequencies, Sturtevant made linkage maps, which are maps of the relative locations, or loci, of genes on a chromosome. On a linkage Biology IDEO V CLIP map, one map unit is equal to one cross-over for each 100 offspring, or one percentage point. You can see an example of a linkage map in figure 3.2. HMDScience.com

GO ONLINE Making a linkage map is fairly easy if all of the cross-over frequencies for the genes being studied are known. Suppose the following data were collected. Gene Mapping • Gene A and gene B cross over 6.0% of the time. • Gene B and gene C cross over 12.5% of the time. • Gene A and gene C cross over 18.5% of the time. According to Sturtevant’s conclusions, genes A and B are 6 map units apart because they cross over 6% of the time. Similarly, genes B and C are 12.5 map units apart because they cross over 12.5% of the time. But where are the genes located in relation to each other on the chromosome?

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FIGURE 3.2 Gene Linkage in Drosophila Linkage maps show the relative locations of genes.

Wild type fruit fly Segment of chromosome 2R Trait Gene (named for mutant phenotype)

wing shape arc 100

eye color brown 102 body size minus 104 bristle size abbreviated

106 wing texture blistered

108 Apply Which genes are most likely to cross over? least likely? Why? (colored SEM; magnification 203) 6 6 A B Think about gene A as a point on a line. Gene B is either to the left or to the A 6 B right of gene A. The same is true of the relationship between genes B and C. A B But if you only know the distances between genes A and B, and between genes 12.5 B 12.5 C B and C, you cannot determine the order of all three genes. You must also 12.5 know the distance between genes A and C. As shown in figure 3.3, the map B C B C distances between genes A and B and between genes B and C equal the map 6 + 12.5 = 18.5 distance between genes A and C. Therefore, gene B must be located between 6 + 12.5 = 18.5 AB6 + 12.5 = 18.5 C genes A and C. If the map distance between genes A and C were 6.5 map units AB C instead of 18.5 map units, then gene A would be between genes B and C. FigureAAB = body 3.3 color The order B = of eye colorC C = wing size genesA = body on a colorchromosome B = eye can color C = wing size Although linkage maps show the relative locations of linked genes, the be determined if all of their A = body color B = eye color C = wing size maps do not show actual physical distances between genes. Linkage maps can cross-over frequencies are known. give you a general idea about distances between genes, but many factors affect gene linkage. As a result, two pairs of genes may be the same number of map units apart, but they may not have the same physical distance between them.

©Dennis Kunkel Microscopy, Inc. ©Dennis Kunkel Microscopy, Summarize How can a linkage map be used to analyze chromosomes? 6h

Self-check Online HMDScience.com 7.3 Formative Assessment GO ONLINE Reviewing Main Ideas Critical thinking CONNECT TO 1. Summarize the importance of 3. Compare and Contrast How are Scientific Process comparing wild type and mutant fruit linked genes similar to sex-linked 5. Punnett, Bateson, and Morgan flies in genetic research. 3F, 6h genes? How are they different? found phenotype ratios that 2. How is a linkage map related to 4. Apply Draw a linkage map based on differed from Mendel’s results. cross-overs that take place during the following cross-over percentages Explain how these differences meiosis? for three gene pairs: A – B = 8%, led to new hypotheses and B – C = 10%, and A – C = 2%. new investigations in genetics. 3F

Chapter 7: Extending Mendelian Genetics 203 CorrectionKey=A DO NOT EDIT--Changes mustbemadethrough “File info” Unit 204 autos ty or Figure 4.1 Figure karyotype pedigree VOCABULARY genomes oforganisms analysis are used to studythe modifications, andchromosomal geneticDNA fingerprinting, 6 non-Mendelian inheritance and crosses, dihybridcrosses and combinations suchasmonohybrid outcomes ofvarious genetic H pe

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Recall from Section 1 that some genetic disorders are caused by autosomal R E ADI N G TO OLB ox genes. A carrier of an autosomal disorder does not show the disease but can pass on the disease-causing allele. Both males and females can be carriers of VOCABULARY an autosomal disorder. The term carrier means “a person who transports In contrast, only females can be carriers of sex-linked disorders. Several something.” In genetics, a genetic disorders are caused by genes on the X chromosome, as you can see in carrier is a person who “transports” a recessive allele figure 4.2. Recall that males have an XY genotype. A male who has a gene for but does not express the a disorder located on the X chromosome will not have a second, normal allele recessive phenotype. to mask it. One copy of the allele is enough for males to have the disorder. There are no male carriers of sex-linked disorders because any male who has the gene displays the phenotype. Females can be carriers because they may have a normal allele that gives them a normal phenotype. The likelihood of inheriting a sex-linked disorder depends both on the sex of the child and on which parent carries the disorder-causing allele. If only the mother has the allele and is a carrier, a child has a 50% chance of inheriting the allele. A daughter who inherits it will not show the phenotype, but a son will. The British royal family provides a historical example of a sex-linked disorder. Queen Victoria (1819–1901) was a carrier of a recessive sex-linked allele for a disorder called hemophilia, which is a lack of proteins needed for blood to clot. People with hemophilia do not stop bleeding easily. Queen Victoria passed the allele to her son, who had hemophilia. He passed it to his daughter, who was a carrier, and so on. Members of royal families tended to marry into royal families in other countries, and by the early 1900s the royal families of several countries, including Russia and Spain, also had the allele for hemophilia. The allele in all of these people is traced back to Queen Victoria. Contrast How can carriers differ between autosomal and sex-linked disorders?

FIGURE 4.2 Comparing the X and Y Chromosomes The X chromosome has about 1100 known genes, including many that cause genetic disorders. The Y chromosome is about one-third the size of the X and has only about 250 known genes.

X Chromosome Examples of known genes Y Chromosome Examples of XY DMD known genes Duchenne’s muscular dystrophy SRY RP2 Testes-determining Retinitis pigmentosa factor DFN2 X-linked deafness TTTY5 Testes-speciﬁc transcript FMR1 Fragile X syndrome OPN1MW Deuteranopia (red-green colorblindness) These X and Y chromosomes F8 are duplicated and condensed. Hemophilia A (colored SEM; magnification about 15,0003) ©Biophoto Associates/Photo Researchers, Associates/Photo ©Biophoto Inc. Researchers,

Chapter 7: Extending Mendelian Genetics 205 DO NOT EDIT--Changes must be made through “File info” CorrectionKey=A

MAIN IDEA 6F A pedigree is a chart for tracing genes in a family. If two people want to know their child’s chances of having a certain genetic CONNECT TO disorder, they cannot rely upon their phenotypes. The parents also need to know their genotypes. A pedigree chart can help trace the phenotypes and Genetic Screening genotypes in a family to determine whether people carry recessive alleles. DNA testing is a direct method of studying genetic disorders. When enough family phenotypes are known, genotypes can often be inferred. You will learn more about DNA A human pedigree shows several types of information. Boxes represent tests and genetic screening in the chapter Frontiers of males and circles represent females. A shaded shape means that a person Biotechnology. shows the trait, a white shape means that the person does not, and a shape that is half-shaded and half-white means that a person is a carrier. Lines connect a person to his or her mate, and to their children. Using phenotypes to figure out the possible genotypes in a family is like putting pieces of a puzzle together. You have to use clues and logic to narrow the possibilities for each person’s genotype. One particular clue, for example, can tell you whether the gene is on an autosome or on a sex chromosome. If approximately the same number of males and females have the phenotype, then the gene is most likely on an autosome. If, however, the phenotype is much more common in males, then the gene is likely on the X chromosome. Tracing Autosomal Genes It is fairly easy to trace genotypes through a pedigree when you know that you are dealing with a trait controlled by an autosomal gene. Why? A person who does not show the phenotype must have a homozygous recessive genotype. Any other genotype—either heterozygous or homozygous dominant—would produce the phenotype. Use the following steps to work your way through a pedigree for a gene on an autosome. The inheritance of an autosomal trait, such as the widow’s peak described earlier, is shown on the top of figure 4.3. • People with a widow’s peak have either homozygous dominant (WW) or heterozygous (Ww) genotypes. • Two parents without a widow’s peak are both homozygous recessive (ww) and cannot have children who have a widow’s peak. • Two parents who both have a widow’s peak can have a child who does not (ww) if both parents are heterozygous for the dominant and recessive alleles (Ww). Tracing Sex-Linked Genes Web When a gene is on the X chromosome, you have to think about the inheritance of the sex chromosomes as well as dominant and recessive alleles. Also, HMDScience.com recall that more males than females show a sex-linked trait in their phenotype GO ONLINE and that females can be carriers of the trait. Genetic Heritage One example of a sex-linked trait is red-green colorblindness. Three genes for color vision are on the X chromosome, so a male with even one recessive allele of one of the three genes is partially colorblind. He will pass that allele to all of his daughters, but he cannot pass the allele to any sons. A pedigree for colorblindness, which is sex-linked, is shown on the bottom of figure 4.3.

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CRITICAL ExplainMale why with it is phenotype not possible to identify all ofFemale the genotypes with phenotype in the pedigree charts VIEWING above. WhatMale information without phenotype would you need to identifyFemale thewithout genotypes phenotype of those people? 6F Male carrier Female carrier Male with phenotype Female with phenotype Chapter 7: Extending Mendelian Genetics 207 Male carrier Female carrier

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Biology Other methods more directly study human chromosomes. HMDScience.com A karyotype (KAR-ee-uh-typ), for example, is a picture of all of GO ONLINE the chromosomes in a cell. In order to study the chromosomes, Human Chromosomes chemicals are used to stain them. The chemical stains produce a pattern of bands on the chromosomes, as shown in figure 4.5. The sizes and locations of the bands are very consistent deletion for each chromosome, but the bands differ greatly among different chromosomes. There- fore, different chromosomes can be easily 1 2 3 4 5 identified in a karyotype. (inset) ©CNRI/Photo Inc. Researchers, Karyotypes can show changes in chromosomes. Chromosome changes can be dramatic,

chromosome 21 such as when a person has too many chromosomes. In Down syndrome, for example, a 6 7 8 9 10 11 12 person has an extra copy of at least part of chromosome 21. In XYY syndrome, a male has an extra Y chromosome. Other times, a karyotype reveals the loss of part of a chromosome. In the figure, you can see a deletion of a large part 13 14 15 16 17 18 of chromosome 1. Scientists also use karyotypes to estimate the distances between genes on a chromosome. A karyotype can help show the 19 20 21 22 possible location of a gene on a chromosome. extra chromosome (inset) ©Christine Harrison/University of UK; Southampton, Chromosome mapping can be done directly by searching for a particular gene. All of the chromosomes are cut X Y apart into smaller pieces. Then this library of chromosome parts is Figure 4.5 A karyotype can help chromosome 1 show chromosomal disorders, searched to find the gene. Although many genes and their locations have been such as the deletion in chromo- identified through this process, it is a slow and inefficient method. The some 1 (top inset) and the extra large-scale mapping of all of the genes on human chromosomes truly began chromosome 21 in Down syn- with the Human Genome Project, an international effort to map, sequence, drome (bottom inset). (LM; magnifi- cations: deletion 80003; colored LM, and identify all of the genes in the human genome. extra chromosome 11,0003) Apply Why must a combination of methods be used to study human genetics?

(r) ©Leonard Lessin/Peter Arnold, (r) Inc.; Arnold, ©Leonard Lessin/Peter Self-check Online HMDScience.com 7.4 Formative Assessment GO ONLINE Reviewing Main Ideas Critical thinking CONNECT TO 1. How can Mendel’s principles be used 5. Apply Suppose a colorblind male Principles of Genetics to study human traits? and a female with no recessive alleles 7. Explain why Mendel’s 2. Is a person who is homozygous for colorblindness have children. principles of inheritance recessive for a recessive genetic What is the probability they will have can be applied to all sexually disease a carrier? Explain. a colorblind son or daughter? reproducing species. 6F 3. Describe how phenotypes can be used to predict genotypes in a pedigree. 6. Contrast How do pedigrees for autosomal genes differ 4. How can a karyotype be used to study from pedigrees for sex-linked genes? human chromosomes? 6H

Chapter 7: Extending Mendelian Genetics 209 CorrectionKey=A DO NOT EDIT--Changes mustbemadethrough “File info” Unit 210

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What isakaryo human chromosomes andto maphumangenes? Punnett’s andBateson’s observations ofpea plants? between alleles? from asimple dominantandrecessive relationship How docodominanc chart? terns for autosomal andsex-linked genes onapedigree What ar How didMorg differ between males andfemales. Describe how thee that dohave thedisease? symptoms of arecessive genetic disease have children Under what circumst phenotype. ment can interact withgenotype to affect anorganism’s Give two e sive alleles onautosomes. auto Explain whydisorders caused by dominantalleles on fol What are two mainways inwhichhumangenetics used t Explain how linked genes andcross-over frequencies are explain thewiderange ofphenotypes? color. How doesthepolygenic nature ofthese traits Humans have atr lows thegenetic patterns seen inother organisms? somes are less common than those caused by reces- o make linkage maps. HMDScience.com Interact 6h 3F e one similarity and one difference between pat-

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211 DO NOT EDIT--Changes must be made through “File info” CorrectionKey=A

Critical Thinking 20. Analyze Can a person be a carrier for a dominant Analyzing Data Construct a Bar Graph genetic disorder? Explain. A scientist studies four linked traits in fruit flies and 21. Apply Both men and women can be colorblind, but observes the frequency with which the traits cross over. there are approximately 100 times as many colorblind The bar graph below shows the cross-over frequencies men as women in the world. Explain why men are more among genes A, B, C, and D. Use the graph to answer the likely to be colorblind than women. next three questions. 22. Apply Suppose two plants with light purple, or laven- Cross-Over Frequencies of Four Genes der, flowers are crossed. About 25% of the offspring 10 10 have white flowers, 25% have purple flowers, and 50% have lavender flowers. Which of the following could 8 8 explain these results: codominance, incomplete domi- 6 6 nance, or multiple alleles? Explain. 6F 4 4 23. Apply Copy the chart below into your science notebook. Following the provided example, fill in the chart to 2 2 0 show the number of possible genotypes given 2, 3, or 4 (%) frequency Cross-over 0 alleles. 6F A–B A–C A–D B–C B–D Gene pairs Number of Alleles Number of Possible Genotypes 2 (A, B) 3 (AA, AB, BB)

3 (A, B, C) 28. Evaluate Which two genes are least likely to be inherited together? How do you know? 4 (A, B, C, D) 29. Evaluate Which two genes are most likely to be 24. Apply Some members of David’s family have an inherited together? How do you know? autosomal recessive disease. David does not have the disease; neither do his parents, nor his two brothers. 30. Apply Explain why a bar graph is an appropriate type of His maternal grandfather has the disease, his paternal graph for displaying these data. grandmother has the disease, and his sister has the disease. Draw a pedigree chart to represent the genotypes of all grandparents, parents, and children. Next to Making Connections each person, write his or her possible genotype. 31. Write an Ad Imagine that you are starting a business 6F that will make pedigree charts for people who want to map particular family traits. Write an ad in simple 25. Synthesize Why are studies of identical twins impor- language that all readers will understand. Your ad should tant in helping understand interactions between envi- demonstrate that you have the necessary understanding ronment and genotype? Explain. of human genetics for a successful pedigree chart- making business. Interpreting Visuals Copy into your science 32. Synthesize Look again at the tremendous range of notebook the pedigree human phenotypes in the photographs on the chapter chart on the right to opener. How is human genetics similar to and different answer the next two from the genetics of Mendel’s pea plants? questions. Use A as the dominant allele, and use a as the recessive allele. 26. Analyze Is this trait most likely recessive or dominant? Explain.

27. Analyze What is the genotype of each individual in the pedigree chart? Explain your answers. 6F

212 Unit 3: Genetics DO NOT EDIT--Changes must be made through “File info” CorrectionKey=B

Biology End-of-Course Exam Practice

Record your answers on a separate piece of paper. 2G, 6E MULTIPLE CHOICE 4 Genes on a pair of chromosomes often cross over during meiosis, as shown in the diagram below. 2G, 6F 1 A genotyped pedigree of 239 people shows evidence of an inheritable recessive disease. One female, however, doesn’t fit the pattern of inheritance demonstrated by all of the other a A a A family members. What is most likely true of this B b b B individual? c C C c A She had been treated for the disease. B She accumulated additional mutations. The discovery of crossing over added to Mendel’s law of independent assortment, which states that C She is immune to the disorder. genes assort independently of one another. What D She was genotyped incorrectly. do crossovers indicate? THINK THROUGH THE QUESTION A Crossing over allows for more than two alleles of a gene. Consider both what a pedigree chart shows and what it doesn’t show. Phenotypes are shown on a pedigree, B Independent assortment between genes but genotypes are inferred from the phenotypes. depends on their locations. C Genes do not assort independently of one another. 6F D Crossing over decreases genetic variation in 2 Many animals, including humans, have sex populations. chromosomes. Which of the following shows the sex chromosome genotype a normal human male 2G, 6F would inherit from his parents? 5 A XX Fruit Fly Dihybrid Cross B YY Eeww\eeWw eW ew eW ew C XY Ew Ew D Y ew ew 6A Red eyes is dominant (E); white eyes is recessive (e). 3 A scientist studying two traits in mice knows that Normal wings is dominant (W); short wings is recessive (w). each trait is determined by one gene and that both genes are on the same chromosome. If the A fruit fly with red eyes and short wings is two traits are not always inherited together by the crossed with a fruit fly with white eyes and offspring of the mice, what is most likely true? normal wings. According to the cross shown in A The genes are not on the same chromosome. the diagram above, what is the expected phenotype of fruit flies in the shaded box? B The genes are far enough apart to allow crossing over. A white eyes and normal wings C The inheritance of these genes leads to B white eyes and short wings codominance. C red eyes and normal wings D One of the genes has a high rate of mutation. D red eyes and short wings

Chapter 7: Extending Mendelian Genetics 213